Inkjet printers, and thermal inkjet printers in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. The operation of such printers is relatively straightforward. In this regard, drops of a colored ink are emitted onto the print media such as paper or transparency film during a printing operation, in response to commands electronically transmitted to the printhead. These drops of ink combine on the print media to form the text and images perceived by the human eye. Inkjet printers may use a number of different ink colors. One or more printheads may be contained in a print cartridge, which may either contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge. An inkjet printer frequently can accommodate two to four print cartridges. The. cartridges typically are mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction above the media during printing such that the cartridges move sequentially over given locations, called pixels, arranged in a row and column format on the media which is to be printed. Each print cartridge typically has an arrangement of printhead nozzles through which the ink is controllably ejected onto the print media, and thus a certain width of the media corresponding to the layout of the nozzles on the print cartridge, can be printed during each scan, forming a printed swath. The printer also has a print medium advance mechanism which moves the media relative to the printheads in a direction generally perpendicular to the movement of the carriage so that, by combining scans of the print cartridges back and forth across the media with the advance of the media relative to the printheads, ink can be deposited on the entire printable area of the media.
The quality of the printed output is a very important feature to purchasers of inkjet printers, and therefore manufacturers of inkjet printers pay a great deal of attention to providing a high level of print quality in their printers. Aberrations in the printhead nozzles can undesirably reduce print quality; such aberrations include, for example, not ejecting ink at all, ejecting an incorrect volume of ink in a drop, producing irregularly shaped drops with artifacts such as tails, or producing a spray of extraneous droplets in addition to the desired drop. Another common type of nozzle aberration is directionality error, also known as dot placement error, in which the drops of ink are not precisely printed in the intended locations on the print media. Different types of printheads can exhibit different types of dot placement errors; these errors are typically due to the design of the printhead and can be characterized for printheads of that particular type. In some types of printheads, it is common for the nozzles located at the top and bottom ends of the printhead to exhibit significant dot placement errors in the direction along the media advance axis, resulting in errors in the swath height, while the nozzles located in the middle of the printhead exhibit less dot placement error. Because the error-prone nozzles print the top and bottom edges of the printed swath in the wrong place, a visually significant print quality defect known as banding results. Banding results in strip-shaped nonuniformities that are visible throughout the printed image.
Banding is more objectionable in areas of the image that contain midtones, rather than highlights (light) or saturated (dark) areas. Dot placement errors are difficult to see in an area of highlights because there is typically so much white space (unprinted areas of the print medium) between the drops of ink that the placement errors are not readily perceived by the human eye. Saturated areas do not exhibit much banding because they contain very little white space, and the large volumes of ink placed in these areas hide most placement errors. But in midtone areas, which have moderate amounts of both white space and ink, small errors in dot placement can have a large effect on how much white space a person perceives.
To minimize banding due to dot placement errors (and coincidentally to also reduce the effect of printing defects resulting from having too much ink on the print medium at one time, such as bleeding of one color area into another and warping or wrinkling of the print media), most printers do not print all the required drops of all ink colors in all pixel locations in the swath in one single scan, or "pass", of the printheads across the media. Rather, multiple scans are used to deposit the full amount of ink on the media, with the media being advanced after each pass by only a portion of the height of the printed swath. In this way, areas of the media can be printed on more than one pass. In a printer which uses such a "multipass" printing mode, only a fraction of the total drops of ink needed to completely print each section of the image is laid down in each row of the printed medium by any single pass; areas left unprinted are filled in by one or more later passes. When printing of a page is complete, every area of the print medium has typically been printed on by the same multiple number of passes. Because each pass uses a different nozzle to print a particular row of the image, multipass printing can compensate for nozzle defects. To illustrate how this compensation works, consider the defect where one particular nozzle in a single-pass printmode does not work at all, causing an unprinted row (or band) of unprinted pixel locations to appear in the printed image. However, if a four-pass printmode is used instead of a single pass, the defective nozzle will only print one out of every four drops in that row, making the impact of the defective nozzle less objectionable. While the above example, for illustration, used a broken nozzle, the same principle applies to nozzles with directionality errors which print ink at incorrect locations.
However, a multipass printmode where all nozzles can deposit the same number of drops of ink is often insufficient to improve print quality to an acceptable level, particularly when specific groups of nozzles have worse errors than other groups, as in the case of swath height error as described above. Therefore, some other approaches to improving print quality have modified the printmode such that all nozzles no longer print the same number of ink drops. For instance, a printmode which prints with only the middle nozzles of a printhead which exhibits swath height error results in improved print quality. However, such an approach has the drawback of significantly increasing the amount of time it takes to print a page, because a smaller swath is printed on each pass.
Another printmode prints with all nozzles, but prints fewer drops from the end nozzles than the middle nozzles. Examples of this printmode is described in the copending and commonly-assigned European patent application Ser. No. 99301151.9, by Vinals, filed Feb. 17, 1999, titled "Printing Apparatus and Method" (Attorney Docket No. 60980088), which is hereby incorporated by reference in its entirety. In order to produce the same print density in all areas of the printed image, however, the edges of each swath must be overlapped by the edges of the next swath, thus reducing the distance the medium can be advanced between passes compared to a printmode in which all nozzles can deposit the same number of drops. As a result of the reduced medium advance distance, the amount of time required to fully print the page is increased. If this technique were applied to a four-pass printmode, for example, the majority of pixel locations on the page will be printed in four passes, but there will be strips of pixels which require five passes to print (known as a "four-five" split-pass printmode).
Just as important to an inkjet printer purchaser as print quality is the amount of time it takes to print a page, or the number of pages that can be printed per unit of time, such as pages per minute. Accordingly, there is still a need for an inkjet printer that minimizes print quality defects due to nozzle aberrations but without significantly reducing printer throughput.